How to change a symlink in OS X?

Try:

unlink /path/to/current/link
ln -s /path/to/target /path/to/symbolic/link

mkdir /path/where/newsymlink
ln -s /path/to/point/to /path/where/newsymlink/is
mv /path/where/newsymlink/is /path/where/symlink/
rmdir /path/where/newsymlink

However, the Java Preferences utility changes more than just a symbolic link; you should use that to ensure that the Java version is changed.

The ln command doesn't let you change links, only create new ones.

Have you compared the permissions on the links and on the targets before and after you change the link? You might just need to follow up with the appropriate chown and chmod commands to get it working.